1. Field of the Invention
This invention relates to manufacture of a magnetoresistive multilayer film utilized for such a magnetic device as giant magnetoresistive (GMR) effect element.
2. Description of the Related Art
The magnetic film technology has been significantly applied to magnetic devices such as magnetic heads and magnetic memories. For example, in magnetic disk drive units for external storages in computers, magnetic heads are mounted for read/write of information. In the field of memory devices, magnetic random access memories (MRAM) utilizing tunnel-type magnetoresistive films for memory elements have been developed. The MRAM are promising next-generation memory devices due to the rapidness of read/write and non-volatility.
In magnetic devices, the magnetoresistive effect is often utilized as means for converting magnetic fields into electric signals. The magnetoresistive effect is the phenomenon that electric resistance is varied according to variation of a magnetic field in a conductor. Especially, magnetic readout heads and MRAM utilize giant-magnetoresistive (GMR) films where the MR ratios are enormously high. “MR ratio” means magnetoresistance ratio, i.e., ratio of electric resistance variation against magnetic field variation. In the field of magnetic recording where further increase of recording density is demanded for enlarging storage capacity, it is necessary to capture slight variation of a magnetic field for reading out stored information. Therefore, the GMR film technology has been utilized in many kinds of magnetic heads, becoming the mainstream.
FIG. 10 is a schematic 3-D view showing the structure of an example of spin-valve type GMR films. The spin-valve type GMR film, hereinafter “SV-GMR film”, has the basic structure where an antiferromagnetic layer 23, a pinned-magnetization layer 24, a nonmagnetic spacer layer (conduction layer) 25 and the free-magnetization layer 26 are laminated in this order. In the SV-GMR film, because the pinned-magnetization layer 24 is adjacent to the antiferromagnetic layer 23, magnetic moment in the pinned-magnetization layer 24 is pinned to a direction by the exchange coupling with the antiferromagnetic layer 23. On other hand, because the free-magnetization layer 26 is isolated from the pinned-magnetization layer 24 by the nonmagnetic spacer layer 25, magnetic moment in the free-magnetization layer 26 is capable of free directions in response to the external magnetic field variation.
The giant magnetoresistive effect on the SV-GMR film derives from spin-dependant scattering of electrons on the interface. When a couple of magnetic layers are magnetized to the same direction, free electrons, i.e., conduction electrons, are easily scattered at the interface. Contrarily, when the layers are not magnetized to the same direction, free electrons are hardly scattered at the interface. Therefore, when the magnetization direction in the free-magnetization layer 26 is closer to the one in the pinned-magnetization layer 24 as shown in FIG. 4, the electric resistance would decrease. When the magnetization direction in the free-magnetization layer 26 is closer to the opposite one to the pinned-magnetization layer 24, the electric resistance would increase. The structure performing this GMR effect is called “spin valve”, because the magnetization direction in the free-magnetization layer 26 is spun against the pinned-magnetization layer 24, which is similar to turning a tap.
Tunnel-type magnetoresistive (TMR) films utilized in the MRAM have MR ratios several times as much as the GMR films. The TMR films are highly expected for next-generation magnetic heads, because of the higher MR ratios. As well as the GMR films, a TMR film has the structure where an antiferromagnetic layer, a pinned-magnetization layer, a nonmagnetic spacer layer and a free-magnetization layer are laminated in this order. The nonmagnetic spacer layer in the TMR film is a very thin film made of insulator, through which a tunnel current flows. Resistance on this tunnel current varies depending on the relative direction of magnetic moment in the free-magnetization layer against the pinned-magnetization layer.
The above-described magnetoresistive multilayer films are manufactured by laminating each thin film for each layer. Each film is deposited by sputtering or another method. In this, what is significant is that the giant-magnetoresistive effect in GMR films and TMR films derives from spin-dependant scattering of electrons on the interface as described. Accordingly, for obtaining a high MR ratio, what is significant is cleanness of the interface between a couple of layers. In depositing a film for a layer on an underlying layer, if a foreign substance is incorporated in the interface or a contaminant layer is formed in the interface, such a fault as MR ratio decrease might be brought. Accordingly, a chamber in which each film for each layer is deposited should be evacuated at a high-vacuum pressure so that the deposition is carried out in the clean environment. In addition, it is significant to shorten the period after the deposition for a layer until the next deposition for the next layer, and to maintain the clean environment continuously in the period.
Interfacial flatness in magnetoresistive multilayer films is also the significant factor in view of enhancing the product performance. Typically, when flatness is worse on the interface of the pinned-magnetization layer and the free-magnetization layer, the interlayer coupling would be generated, decreasing the product performance. This point will be described in detail as follows, referring to FIG. 11.
FIG. 11 shows the mechanism of the interlayer coupling generated from the worsened flatness of an interface. It is assumed in FIG. 11 that the magnetization layer 24 is formed as its surface is much roughened. This results in that the nonmagnetic spacer layer 25 and the free-magnetization layer 26 are also formed with the surfaces much roughened. If each surface of each layer 24,25,26 is completely flat, theoretically no magnetic poles would be induced in the interfaces. Contrarily, magnetic poles would be easily induced if the interfaces are roughened. For example, the magnetic lines in the angles of the roughened pinned-magnetization layer 24 generate poles at the ends because they terminate on the slopes of the angles. In the free-magnetization layer 26, the magnetic lines in the roots thereof generate poles at the ends.
When magnetic poles are induced on the interface between the pinned-magnetization layer 24 and the free-magnetization layer 26 as described, the interlayer coupling would take place between them, in spite of isolation by the nonmagnetic spacer layer 25. As a result, magnetic moment in the free-magnetization layer 26 would be captured by the pinned-magnetization layer 24, being incapable of the free rotation. If this happens, for example, in a magnetic readout head, readout signals would be asymmetrical against variation of the external magnetic field (the magnetic field on a storage medium). Otherwise, response of the readout head would be delayed to variation of the external magnetic field. These results might cause kinds of readout errors. In a MRAM, it might cause kinds of write-in errors and readout errors. It could also happen that magnetization direction in the free-magnetization layer 26 does not vary against magnetization direction in the pinned-magnetization layer 24 even when the external magnetic field varies. Therefore, the MR ratio tends to decrease when flatness of the interface is worsened.
The problems of the interlayer coupling and the interfacial roughness are discussed in J. Appl. Phys., Vol. 85, No. 8, p 4466-4468. This paper describes roughness is generated from the growth structure of a film. J. Appl. Phys., Vol. 7, No. 7, p 2993-2998 describes roughness of a film would be promoted when pressure in depositing the film is increased. After all, these papers teach that to decrease pressure in depositing a film is effective to make the interfacial roughness small for reducing the interlayer-coupling. However, J. Appl. Phys., Vol. 77, No. 7, p 2993-2998 also points out that intermixing, which means mutual incorporation of materials through an interface, would take place when pressure in depositing a film is decreased.
As another solution for the problem of the interlayer coupling caused by the interfacial roughness, it is considered to thicken the nonmagnetic spacer layer. However, when the nonmagnetic spacer layer is thickened in the SV-TMR film, the flow of conductive electrons not contributing to the GMR effect would be promoted, causing the problem of decreasing the MR ratio. The flow of those electrons is called “shunt effect”. In the TMR film, on the other hand, if the nonmagnetic spacer layer of insulator is thickened, the whole resistance is increased, resulting in that the optimum tunnel current could be no longer obtained. This would cause the problem of decreasing the product performance.
There is still a further way to reduce roughness of an interface, which is shown in the Japanese laid-open No. 2003-86866. In this way, after the film deposition for a layer is carried out, the surface of the deposited film is treated utilizing plasma before the next film deposition for the next layer. However, an apparatus for this way accompanies the problem of scale enlargement because equipment for the plasma treatment is required. In addition, the problem of decreasing the productivity is also accompanied because the extra step of the plasma treatment is required.